Streptococcus pneumoniae is one of the largest causes of death by infectious disease of the elderly worldwide. It is also one of the greatest causes of death among very young children in developing countries. Although the polysaccharide-protein conjugate vaccine is effective at protecting children from bacteremia and sepsis, protection is restricted to the included capsular types. The vaccine is only partially protective against otitis media and its potential for herd immunity is limited. Moreover, the conjugate vaccine is more than 100-fold too expensive for widespread use in the developing world. One way to improve this vaccine, or possibly replace it, would be to use protection-eliciting cross-reactive proteins of pneumococci. Several such proteins have been identified, and one, PspA, has reached clinical trials. PspA is required for full virulence of pneumococci in mice and antibodies to it are protective against sepsis, pneumonia, and carriage. Antibody to PspA can enhance the clearance of pneumococci from the blood of infected animals, and it appears to be able to increase complement deposition on the pneumococcal surface in vitro. Antibodies to PspA promote the attachment of pneumococci to phagocytes, but have not been found to be opsonic (even in the presence of complement) for phagocytosis and killing in vitro. To obtain a better understanding of how antibodies to PspA promote protection in vivo, we will examine several of their known biologic effects in detail. Our investigations will include in vitro conditions that are as close as possible to those in vivo. Investigations of a panel of protective and non-protective monoclonal antibodies, all of which will react with native PspA, will allow us to determine which biologic assays are relevant to in vivo protection. We will also map the epitopes that elicited the monoclonal antibodies. Identification of the in vivo mechanism by which antibody to PspA protects should enable development of a valid surrogate assay for protection, and should improve our understanding of pneumococcal disease. Our analyses of the protection-eliciting and non-protection-eliciting epitopes on PspA should make it possible to design even better PspA vaccines in the future.